Brushless direct current (BLDC) motors are used in industries such as appliances, automotive, aerospace, consumer, medical, industrial automation equipment and instrumentation. BLDC motors do not use brushes for commutation, instead, electronic commutation is used. BLDC motors have advantages over brushed DC motors and induction motors such as: better speed versus torque characteristics, high dynamic response, high efficiency, long operating life, longer time intervals between service, substantially noiseless operation, and higher speed ranges. More detailed information on BLDC motors may be found in Microchip Application Notes: AN857, entitled “Brushless DC Motor Control Made Easy,” (2002); AN885, entitled “Brushless DC (BLDC) Motor Fundamentals,” (2003); AN894, entitled “Motor Control Sensor Feedback Circuits,” (2003); AN901, entitled “Using the dsPIC30F for Sensorless BLDC Control,” (2004); and AN970, entitled “Using the PIC18F2431 for Sensorless BLDC Motor Control,” (2005); all are hereby incorporated by reference herein for all purposes.
A three-phase BLDC motor completes an electrical cycle, i.e., 360 electrical degrees of rotation, in six steps at 60 electrical degrees per step. Synchronously at every 60 electrical degrees, phase current switching is updated (commutation). However, one electrical cycle may not correspond to one mechanical revolution (360 mechanical degrees) of the motor rotor. The number of electrical cycles to be repeated to complete one mechanical revolution depends upon the number of rotor pole pairs. For example, a four-pole BLDC motor will require two electrical cycles to complete one mechanical revolution of the motor rotor (see FIG. 3).
Drive commutation for a BLDC motor may be determined by monitoring the back electromotive force (EMF) voltages at each phase (A-B-C) of the motor. The drive commutation is synchronized with the motor when the back EMF of the un-driven phase crosses one-half of the motor supply voltage during a commutation period. This is referred to as “zero-crossing” where the back EMF varies above and below the zero-crossing voltage over each electrical cycle. Zero-crossing is detected on the un-driven phase when the drive voltage is being applied to the driven phases. A voltage polarity change about the zero-crossing voltage of the back EMF on the un-driven phase may also be used in detecting a zero-crossing event, e.g., from positive to negative or negative to positive during application of the drive voltage to the driven phases within certain limits.
When driving sensorless brushless DC motors, large gaps occur in the drive voltage at high motor rotational speeds and 50% or less PWM duty cycle. The PWM drive control algorithm becomes unstable when the uncertainty of zero-crossing detection reaches about 20%. Gaps in drive voltage create timing errors in the zero-crossing detection, and high PWM frequencies are inefficient and lead to power field effect transistor (FET) failures due to overheating of the FET power driver.
Drive commutation is synchronized with the sensorless BLDC motor when the back electromotive force (BEMF) of the un-driven phase crosses half the motor supply voltage in the middle of the commutation period. This is sometimes referred to as zero-crossing. Zero-crossing is only valid when the drive voltage is applied to the other two phases. Drive voltage is varied by pulse width modulating the full drive voltage. Therefore, during PWM drive off periods zero-crossing cannot be detected. When the PWM frequency is low and the motor speed is high, the low (off) PWM periods can be a significant percentage of the commutation period and thereby may cause a gap in the zero-crossing detection. When these gaps are more than 20 percent of the commutation period they will cause instability in the control algorithm. The gap percentage can be reduced by increasing the PWM frequency at the expense of increased and undesirable switching losses.